Novel well-defined hydrophilic cationic polymers with different length of polycation chain for gene delivery, poly(vinyl pyrrolidone)-graft-poly(2-dimethylaminoethyl methacrylate)s (PPDs), were synthesized by atom transfer radical polymerization (ATRP). The chemical structures and compositions of these polymers were characterized by FT-IR, (1)H-NMR and GPC. The experimental results of dynamic light scattering (DLS), ζ-potential and transmission electron microscopy (TEM) indicated that PPD could condense plasmid DNA (pDNA) to form nanocomplexes. Agarose gel retardation assays demonstrated that PPD could encapsulate plasmid DNA completely when the N/P ratio is equal to or above 3. MTT assay and in vitro gene transfection results indicated that PPD/pDNA complexes exhibited high transfection efficiency concomitant with obvious cytotoxicity. Furthermore, bovine serum albumin (BSA) was utilized to assembly with the binary complexes of PPD/pDNA to screen the residual surface positive charges of complexes in order to decrease cytotoxicity of the binary complexes. Physicochemical properties were characterized and the results indicated that the coating of BSA was able to decrease the zeta potential of the nano-sized PPD/pDNA complexes nearly to electroneutrality without interfering with DNA condensation ability. The ternary complexes of BSA/PPD/pDNA demonstrated no cytotoxicity and also maintained high gene transfection efficiency in HepG2 cells in 10% serum compared with that in serum-free condition.